Blending polymers of widely different properties offers a convenient method of arriving at new compositions which provide a unique set of properties tailored for specific applications. For example, copolymers of acrylates and acrylic acid have good low temperature flexibility and general adhesive characteristics, but for many applications, such as in a hot melt adhesive, a higher melting point, as is frequently found in crystalline polymers, is desirable. Generally, however, blends of acrylate-acrylic acid copolymers are not miscible with crystalline polymers which have melting points in the range desired for hot melt adhesives. Preparation of such blends usually leads to poor mechanical compatibility or gross phase separation, which is to be expected with immiscible polymer blends.
Attempts have been made to solve this problem of immiscibility by promoting some chemical bonding between the blended polymers. For example, U.S. Pat. No. 3,984,497 Owens et al (1976) discloses forming a multi-phase emulsion polymerized polymer having an elastomeric phase, such as a butadiene-styrene copolymer, and a rigid phase containing amine-reactive carboxylic groups, such as a copolymer of an alkyl methacrylate and acrylic or methacrylic acid. Such acrylic elastomer-based interpolymers are blended and extruded with a polycarbonamide, such as nylon 6, to improve impact strength of the latter. It is stated that the amine-reactive groups of the multi-phase polymer graft to the polycarbonamide by reacting with an amine end group of the latter. The polycarbonamide is thus modified to render it extrudable, blow moldable and injection moldable.
U.S. Pat. No. 4,035,438, Niellnger, et al. (1977), discloses impact resistant polymer mixtures of polyethylene, poly-.epsilon.-caprolactam and a graft polymer having an ethylene/(meth)acrylic acid substrate carrying graft polymerlzed units of .epsilon.-caprolactam. U.S. Pat. No. 4,160,790, Mason, et al. (1979), discloses improving the impact strength of nylon 6 or 66 by melt-blending the nylon with both an ethylene/acrylic acid copolymer and an ethylene/ethyl acrylate copolymer.
U.S. Pat. No. 4,247,665, Daniels, et al. (1981), discloses molding compositions of an aliphatic polyamide, such as nylon 6, 66 or 11, modified with a copolymer made by blending nylon 6 with an acrylic multi-phase polymer having an elastomerlc phase and a rigid phase and derived from acrylonitrlle or an alkyl (meth)acrylate and a copolymerizable unsaturated carboxylic acid. The modifier polymer is prepared so that some of the carboxylic acid groups of the acrylic polymer react with some of the amine end groups of the nylon 6.
U.S. Pat. No. 4,404,325, Mason, et al. (1983), discloses high impact resistant nylon made by dry blending and melt extruding nylon e.g., poly(.epsilon.-caprolactam), a copolymer of ethylene and a methyl or ethyl acrylate ester, and an ionic copolymer, such as a copolymer of ethylene and an unsaturated carboxylic acid, such as acrylic acid.
On the other hand, others have attempted to reduce the chemical interaction between blended polymers by neutralizing interactive groups. For example, U.S. Pat. No. 4,310,638, Coran, et al. (1982), discloses an elastomeric composition of nylon blended with an acrylic copolymer of ethylene, alkyl acrylate and unsaturated carboxylic acid wherein at least 50% of the acid groups in the copolymer are neutralized with metal ions before blending with the nylon. Magnesium oxide is given as a suitable neutralizing agent.
Hot melt adhesives is one area where the potential of polymer blends has offered the possibility of property improvements. Hot melt adhesives based on polyamides are thermoplastics formed by the reaction of dicarboxylic acids with diamines. The dicarboxylic acids used in these polyamides are unsaturated fatty acids often referred to as dimer acids, which consist of mixtures of high molecular weight (for example, 30 to 40 carbon atoms per molecule) based dicarboxylic acids usually containing several isomeric dimers along with minor amounts of timers. These dimer acids are condensed with diamines, such as ethylenediamine and hexamethylenediamine. Such products which are available commercially generally have molecular weights in the range of 2,000 to 15,000 and exhibit softening points between 25.degree. C. and 200.degree. C., depending on the structure and molecular weight of the polymer. Such polyamides can be improved for the purpose of use as hot melt adhesives by polymer blending. For example U.S. Pat. No. 4,018,733 Lopez, et al. (1977), discloses a hot melt adhesive formed by mixing a polyamide and an acidic ethylene polymer. The polyamide is made by copolymerizing a diamine and a dibasic acid, and the ethylene polymer is a copolymer of ethylene and an aliphatic ester of an unsaturated mono- or dicarboxylic acid.
U.S. Pat. No. 4,132,690, Eernstman, et al. (1979), discloses that polyamide hot melt adhesives are improved in adhesive properties by blending with a copolymer of ethylene and an unsaturated carboxylic acid having an acid number of at least 100 and a molecular weight of 500 to 5,000.
It is usual in forming hot melt adhesives with polyamides to specify a particular type of polyamide in order to solve compatibility problems and achieve a sufficiently low molecular weight. For example, U.S. Pat. No. 4,374,231, Doucet (1983), describes a hot melt adhesive of polyamide blended with a terpolymer of ethylene, acrylic acid and an alkyl acrylate. The polyamides are based on one or more dimeric fatty acids and have ring-and-ball softening points preferably of 70.degree.-135.degree. C.
More recently, it has been disclosed that polyamides have found utility in pressure sensitive adhesives. For example, U.S. Pat. No. 4,769,285, Rasmussen (1988), describes a pressure sensitive adhesive formed by solution blending a polyamide with an acrylic polymer, such as a copolymer of isooctyl acrylate and acrylic acid.
Although it has been known for almost two decades that .epsilon.-caprolactam could be grafted to a polyethylene-acrylic acid copolymer; for example, see Matzner, et al., Polym. Sci. & Technol., Vol. 6, page 134 (1972), this approach has not been used in modifying the properties of polymers for hot melt adhesives. Matzner, et al. disclose simply the preparation of an ethylene/acrylic acid/nylon 6 graft terpolymer by heating the mixture of the .epsilon.-caprolactam and the copolymer. Low molecular weight ethylene-acrylic acid copolymers are used for hot melt adhesives, but the upper temperature limit of utility is generally 90.degree. C. due to the crystalline melting point of the ethylene sequences.